1. Field of the Invention
The present invention relates to a semiconductor acceleration sensor and particularly to a semiconductor acceleration sensor which may be used, for example, in suspension control and anti-lock braking system (ABS) for vehicles.
2. Description of the Related Art
FIG. 5 and FIG. 6 show, in a side sectional view and a perspective view, respectively, a conventional semiconductor acceleration sensor. The sensor has a cap, which will be described later, the cap being illustrated in FIG. 5 but not illustrated in FIG. 6. Referring to these drawings, the sensor has an acceleration sensing bar or beam 1. Gauge resistors 2, utilizing the piezoresistive effect of a semiconductor, are formed at a certain position of the acceleration sensing beam 1 on one of a pair of opposed surfaces of the beam 1 (on an upper surface, as viewed in FIGS. 5 and 6). The gauge resistors 2 are connected with each other to form the bridge of a bridge circuit. A diaphragm 3, i.e., a thinner portion of the beam, is formed at a position of the acceleration sensing beam 1 where the gauge resistors 2 are located by reducing the thickness of the beam 1 from the other surfaces (from a lower surface, as viewed in FIGS. 5 and 6). A first end of the acceleration sensing beam 1 is cantilevered by a footing 4 serving as the support at the second end of the cantilever structure.
A weight 5 for improving sensitivity is mounted to the first end of the acceleration sensing beam 1 with an adhesive. The footing 4 is attached to a base 6 serving as a part of an enclosure of the semiconductor acceleration sensor. A cap 7 serving as the other part of the enclosure of the sensor is mounted on the base 6 in such a manner as to define the interior of the device. Lead pins 8 for inputting and outputting electrical signals are retained by the base 6 by using an insulator, not shown. The acceleration sensing beam 1 and the lead pins 8 are electrically interconnected by wires 9.
In the conventional semiconductor acceleration sensor having the above construction, when an acceleration is applied to the sensor, the end portion of the acceleration sensing beam 1 having the weight 5 thereon swings vertically about the fulcrum, i.e., on the footing 4, whereby the beam 1 is deflected. The acceleration sensing beam 1 deflects at the thinner portion 3 so that stresses concentrate at that thinner portion 3 with the result that the resistances of the gauge resistors 2 change accordingly by virtue of the piezoresistive effect of the semiconductor. The changes in the resistances cause an imbalance in the bridge circuit including the gauge resistors 2, generating a potential difference, which is detected as the magnitude of the acceleration.
With the above-described conventional semiconductor acceleration sensor in the event that the weight 5 is separated from the beam 1 during operation, it is not possible to sense the separation since the gauge resistors 2 are only electrically connected to an external circuit. When the weight 5 is separated, sensitivity becomes very low, thereby involving the risk that acceleration applied to the sensor may not be properly sensed, thus resulting in erroneous operations.